KUALA LUMPUR, Malaysia (AP) — The international search for the missing Malaysian jetliner expanded further into the Indian Ocean on Friday amid signs the aircraft may have flown on for hours after its last contact with air-traffic control nearly a week ago.

A U.S. official told The Associated Press that the Malaysia Airlines plane sent signals to a satellite for four hours after the aircraft went missing early last Saturday, raising the possibility the jet carrying 239 people could have flown far from the current search areas. It also increased speculation that whatever happened to the plane was a deliberate act.

Malaysia’s Transport Minister Hishammuddin Hussein said the search was expanding further afield, not because of any new information about the plane’s flight, but because the aircraft has not yet been found.

Malaysian officials declined to discuss when —or even whether — they had information about signals to satellites, and that they would release details only when verified. Hishammuddin said Malaysian investigators have worked with U.S. colleagues in Kuala Lumpur since Sunday.

“I hope within a couple of days to have something conclusive,” he told a press conference.

If the plane had disintegrated during flight or had suffered some other catastrophic failure, all signals — the pings to the satellite, the data messages and the transponder — would be expected to stop at the same time. Experts say a pilot or passengers with technical expertise may have switched off the transponder in the hope of flying undetected.

No theory, however, has been ruled out in one of aviation history’s most puzzling mysteries.

The Beijing-bound Boeing 777-200 last communicated with air traffic base stations east of Malaysia in the South China Sea, which for several days has the main focus of the search. Planes and ships also have been searching the Strait of Malacca west of Malaysia because of a blip on military radar suggested the plane might have turned in that direction after the last confirmed contact.

If the plane flew another four hours, it could be much farther away.

Indian ships and planes have been searching northwest of Malaysia in the eastern Andaman Sea, and on Friday expanded their search to areas west of the Andaman and Nicobar Islands chain Friday, said V.S.R. Murty, an Indian Coast Guard inspector-general.

The White House said the U.S. may be drawn into a new phase of the search in the vast Indian Ocean but did not offer details. The U.S. Navy 7th Fleet said it was moving one of its ships, the USS Kidd, into the Strait of Malacca.

Hishamuddin said the search also would be expanded to more remote parts of the South China Sea as well.

However, Vietnam, which has been heavily involved in the search from the start, downgraded its hunt in the South China Sea to regular from emergency on Friday by reducing the frequency of aircraft flights and cruises by ships involved, said Lt. Gen. Vo Van Tuan, deputy chief of staff of Vietnamese People’s Army.

The U.S. official, who spoke on condition of anonymity because he wasn’t authorized to discuss the situation by name, said the plane wasn’t transmitting data to the satellite but was sending a signal to establish contact.

Boeing offers a satellite service that can receive a stream of data during flight on how the aircraft is functioning and relay the information to the plane’s home base. The idea is to provide information before the plane lands on whether maintenance work or repairs are needed.

Malaysia Airlines didn’t subscribe to that service, but the plane still had the capability to connect with the satellite and was automatically sending pings, the official said.

Boeing has not commented.

“It’s like when your cellphone is off but it still sends out a little ‘I’m here’ message to the cellphone network,” the official said. “That’s how sometimes they can triangulate your position even though you’re not calling because the phone every so often sends out a little bleep. That’s sort of what this thing was doing.”

Messages involving a different, more rudimentary data service also were received from the airliner for a short time after the plane’s transponder — a device used to identify the plane to radar — went silent, the U.S. official said.

Hishammuddin said Malaysia was asking for radar data from India and other neighboring countries to see if they can trace it flying northwest. There was no word Friday that any other country had such details on the plane, and they may not exist.

In Thailand, secondary surveillance radar, which requires a signal from aircraft, runs 24 hours a day, but primary surveillance radar, which requires no signal at all, ordinarily shuts down at night at some locations, said a Royal Thai Air Force officer who asked not to be named because he is not authorized to talk to the media on the issue.

Air Marshal Vinod Patni, a retired Indian air force officer and a defense expert, said radar facilities in the Andaman and Nicobar Islands area don’t work around the clock, either. “These are generally switched on and off as and when required. A radar may be kept on for 24 hours on certain days. I won’t say that the Indian radars are highly sophisticated in the region,” he said.

Patni also said there are gaps in the coverage areas, including within the area being searched for the missing plane. He couldn’t give an exact location for specific gaps, but said pilots are well aware of them.

The possibility that the plane flew long after its last confirmed contact opens the possibility that one of the pilots, or someone with flying experience, wanted to hijack the plane for some later purpose, kidnap the passengers or commit suicide by plunging the aircraft into the sea.

Mike Glynn, a committee member of the Australian and International Pilots Association, said he considers pilot suicide to be the most likely explanation for the disappearance, as was suspected in a SilkAir crash during a flight from Singapore to Jakarta in 1997 and an EgyptAir flight from Los Angeles to Cairo in 1999.

“A pilot rather than a hijacker is more likely to be able to switch off the communications equipment,” Glynn said. “The last thing that I, as a pilot, want is suspicion to fall on the crew, but it’s happened twice before.”

Glynn said a pilot may have sought to fly the plane into the Indian Ocean to reduce the chances of recovering data recorders, and to conceal the cause of the disaster.

Experts said that if the plane crashed into the ocean, some debris should be floating even if most of the jet is submerged. Past experience shows that finding the wreckage can take weeks or even longer, especially if the location of the plane is in doubt.